Well, you may have obtained a custom upgraded Server model Late 2012 i7 2.3GHz quad-core
however they shipped (Server stock) with two 1-TB hard disk drives, of 5400-RPM each. As mine.
The 'About this Mac' + More Info button brings up 'System Information' window with select options:

This is how standard configuration Late 2012 i7 Server Mac Mini 2.3GHz appears with dual 1-TB HDs.
The drives are mechanically identical, and you should be able to tell within the system and by drive
serial number among other things, what they are. However IF you did get an uncharacteristic model
of Mac Mini 'Late 2012' Server i7 2.3GHz quad-core (that does not have) two 1- TB hard disk drives:
Likely you will have to backup your content you wish to keep, on external enclosed hard drive storage;
and use the OS X Recovery version of OS X Utilities, to reformat a single 2- TB HDD to make it one
partition again. Then use Online Recovery to attempt to restore all partitions (including local Recovery
partition that helps you choose how to restore your computer) if you did not create a quality backup
clone that included all of the partitions which main HDD had when new. Many are hidden in daily use.
You could use Time Machine and OS X Recovery to attempt to return the erased reformatted hard drive
to a previously usable single partition hard drive, if you have a suitable backup in Time Machine.
• Use Time Machine to back up or restore your Mac - Apple Support
• OS X: About OS X Recovery - Apple Support
'Recovery Disk Assistant' can be used to create a Recovery System on external drive that has same capability
as the built-in Recovery System; if done in advance of the requirement or necessity, as a precaution.
• OS X: About Recovery Disk Assistant - Apple Support
• OS X Tips - Using the Recovery HD
http://pondini.org/OSX/RecoveryHD.html
• Apple OS X and Time Machine Tips:
http://pondini.org/OSX/Home.html
Sorry of any confusion in this matter; excepting my interest in the actual version of Server edition I own and
am using while replying to your topic -- which I studied a year before getting mine last year direct from Apple
online -- otherwise I'd have not known configuration details as-shipped, optional; & standard Server, as mine.
However, you should look into the actual cause of slowness, since your drive information suggests there
really isn't much capacity used in either partition (or apparent hard drive) so that should not be slow unless
you have installed some anti-virus or other application is using CPU cycles excessively; or hogging RAM.
To resolve the actual cause of this memory issue can be done on the present configuration. The Etrecheck
report, if made and posted into a discussions window, could be helpful for others to see it & offer advice.
In any event, hopefully you can restore your server Late 2012 i7 without much fuss...
Good luck & happy computing!